This invention relates an optical waveguide device and its fabrication method applicable for long haul and large capacity optical fiber communication system and an optical measuring instrument, particularly to an optical waveguide device having a metal-cladding type optical polarizer and its fabrication method for achieving a high extinction ratio and stabilizing the device performance against fluctuation of incident lightwave.
An optical waveguide device comprising an optical waveguide, a buffer layer and electrodes which are fabricated on a substrate having electro-optic effect, such as lithium niobate (LN), has been widely employed for long haul and large capacity optical fiber communication system and an optical measuring instrument.
The optical waveguide device is fabricated as follows: First of all, the optical waveguide is formed on the LN substrate by a Ti thermal diffusion method, and the buffer layer made of SiO2, and the electrodes for modulation of lightwave propagating through the optical waveguide, are successively fabricated on the substrate. Then, the substrate is diced into chips and they are mounted on cases. Finally, optical fibers are connected to both ends of each chips to complete optical waveguide devices.
In general, the optical waveguide devices are operated using the lightwave with perpendicularly or horizontally polarized to a main surface of a substrate. From this point of view, a polarization maintaining fiber is usually connected to the input of the optical waveguide of the device, and the linearly polarized lightwave is introduced into the optical waveguide device.
However, if the polarization maintaining fiber is stressed from the outside or non-linearly polarized lightwave is introduced, extinction ratio of the optical waveguide device would be degraded. As the result, the optical waveguide device can not do on/off switching of lightwave correctly, so that the SIN ratio is degraded in the signal transmittance.
To solve the above-mentioned problem, a metal-cladding type optical polarizer proposed by Suematsu et al. in xe2x80x9cAppl. Phys. Lett., Vol. 21, No. 6 (1972)xe2x80x9d is applicable for the optional waveguide device. The metal-cladding type optical polarizer is utilizing difference in degree of electric field absorption depending on the state of polarization. Concretely, by the metal-cladding type optical polarizer, only polarized lightwave horizontal to a main surface of a substrate (TE mode lightwave) is transmitted, and a polarized lightwave perpendicular to the main surface (TM mode lightwave) is absorbed. The above optical waveguide device can exhibit a relatively high extinction ratio, regardless of its simple structure.
For the reduction of the excess absorption of the TE mode lightwave and the achievement of high extinction ratio, a dielectric film having a lower refractive index than that of the core of the optical waveguide may be fabricated as an intermediate layer between the optical waveguide and the metal-cladding type optical polarizer.
On the other hand, for the improvement of velocity matching, a buffer layer made of a dielectric material, such as SiO2, is fabricated between the substrate and the electrodes to modulate the lightwave propagating through the optical waveguide, In this case, it is proposed that the buffer layer is partially etched by reactive ion etching using a fluorine-based gas such as CF4 or CHF3 to form an opening so as to expose the part of the substrate, and the metal-cladding type optical polarizer is provided in the opening. Therefore, the metal-cladding type optical polarizer can be integrated on the optical waveguide device having a thick buffer layer.
However, in this case, the buffer layer may be over-etched due to the fluctuation in its thickness and/or in the etching rate corresponding to different etching batches, so that the substrate may be partially etched. Therefore, if the metal-cladding type optical polarizer is provided on the etched substrate in the opening, the polarization property of the metal-cladding type optical polarizer may be fluctuated according to degree of over-etching against the substrate, resulting in the deterioration of the extinction ratio of the optical waveguide device.
On the other hand, the metal-cladding type optical polarizer may be fabricated on the intermediate layer composing by dielectric materials. In this case, during the long-term use of the optical waveguide device, mechanical peeling of the metal-cladding type optical polarizer due to heat cycle and/or thermal shock can be avoided.
However, when the metal-cladding type optical polarizer is fabricated on the intermediate layer composing by oxides, such as SiO2, it is oxidized and corroded by long-term use. As a result, the performance of the optical waveguide device as having the metal-cladding type optical polarizer can not be stabilized for a long time.
It is an object of the present invention to provide an optical waveguide device with a metal-cladding type optical polarizer and its fabrication method to be able to realize a high extinction ratio for a long time through the performance of the metal-cladding type optical polarizer, irrespective of its configuration being formed on an intermediate layer or in an opening at the buffer layer, with keeping its formation advantages.
For achieving the above object, this invention relates to an optical waveguide device comprising:
a substrate made of a material with an electro-optic effect,
an optical waveguide formed on the substrate to transmit a lightwave,
a modulation electrode to modulate the lightwave,
a metal-cladding type optical polarizer to remove the unnecessary polarized component of the lightwave, and
an intermediate layer, between the substrate and the metal-cladding type optical polarizer, made of at least one dielectric material selected from the group consisting of AOx, B2Oy, COz (A: divalent element, B: trivalent element, C: quadrivalent element, 0 less than x less than 1, 0 less than y less than 3, 0 less than z less than 2, O: oxygen).
The inventors have intensely studied to stabilize the performance of an optical waveguide device having a metal-cladding type optical polarizer, and then, investigated the structure of the optical waveguide device in detail.
As a result, they found that the metal-cladding type optical polarizer was not peeled if it was fabricated on an intermediate layer with low internal stress, as mentioned above.
Since the metal-cladding type optical polarizer is required to be formed on the optical waveguide, the intermediate layer must be formed on the same optical waveguide. Therefore, if the intermediate layer is made of a material with higher refractive index than that of the substrate or the optical waveguide, a lightwave propagating through the optical waveguide is leaked toward the intermediate layer. Thus, the intermediate layer is required to be made of a material with relatively lower refractive index such as dielectric oxides.
As mentioned above, however, if the metal-cladding type optical polarizer is provided on the intermediate layer composing by dielectric oxides, it is oxidized and corroded. Therefore, the inventors have precisely studied to clarify the mechanism of the oxidization and corrosion. As a result, they have found that the excess oxygen in the intermediate layer was extricated with time, and diffused into the metal-cladding type optical polarizer formed on the intermediate layer, thereby to oxidize and corrode the polarizer.
Based on the above-mentioned study, the inventors have made a investigation to inhibit the extrication of the excess oxygen. As a result, they have found that oxidization and corrosion of the metal-cladding type optical polarizer could be inhibited when the intermediate layer was made of a dielectric oxides having a lower oxygen concentration than that of its stoichiometric concentration, because the excess oxygen was not generated in the intermediate layer.
This invention is achieved by the extensive and detailed researches as mentioned above.
The optical waveguide device of the present invention is created directly from the above researches, and has the intermediate layer of dielectric oxide between the substrate and the metal-cladding type optical polarizer.
According to the present invention, mechanical peeling, oxidization and corrosion of the metal-cladding type optical polarizer can be avoided, and thus, the performance of the optical waveguide device can be stabilized for a long time.
Furthermore, this invention relates to a fabrication method of an optical waveguide device consisting of an optical waveguide, a buffer layer, a metal-cladding type optical polarizer and a modulation electrode on a substrate with an electro-optic effect, comprising:
a first step to form the optical waveguide on the substrate,
a second step to fabricate the buffer layer on the substrate,
a third step, by means of partial removal of the buffer layer employing non-reactive dry-etching, to form a first opening so that the corresponding part of the main surface of the substrate on which the optical waveguide is formed, is partially or entirely exposed, and
a fourth step to fabricate the metal-cladding type optical polarizer on the substrate inside the first opening.
Also, the inventors investigated, for a LN substrate, the cause of fluctuation in optical performance of the metal-cladding type optical polarizer, when it was fabricated in a buffer layer opening of which the substrate was slightly etched in the formation process.
As the result, they found that the metal-cladding type optional polarizer was corroded when it was fabricated in such opening, although it was not corroded when it was fabricated directly on the substrate without forming buffer layer. As they made further investigation on the corrosion phenomenon, they found that the excess oxygen from the main surface of the substrate was the cause of its corrosion.
Then, since the excess oxygen is generated when the substrate is attacked by fluorine-based gas plasma because of over-etching of the buffer layer, the inventors conceive the following mechanism of generating the excess oxygen. That is, when the chemically reactive etching using fluorine-based gas plasma is employed, the fluorine radicals as main etching species are selectively reacted with the lithium constituting the LN substrate, to form fluorides. Because, the lithium is selectively extricated from the LN substrate as lithium fluoride, the excess oxygen is generated.
On the above excess oxygen generating mechanism, the inventors employ non-reactive physical etching using argon gas plasma, and the buffer layer is partially etched to form an opening, in which the metal-cladding type optical polarizer is fabricated In this case, the metal-cladding type optical polarizer is never corroded, and thus, the optical performance of the polarizer is not changed.
This invention is also achieved through the vast research and development.
According to the present invention, even though the substrate is attacked by fluorine-based gas plasma because of over-etching of the buffer layer occurred in the fabrication process of buffer layer opening, the excess oxygen is not generated on the main surface, particularly in the superficial layer of the substrate. Therefore, when the metal-cladding type optical polarizer is fabricated in the opening, it is not oxidized and corroded.
Accordingly, the polarization property of the metal-cladding type optical polarizer can be stabilized and thus, the optical waveguide device having a high extinction ratio can be provided.